1. Field of the Invention
This invention relates to digital telecommunications and more particularly, to a novel bi-directional 3-way branching apparatus to drop and insert service channels in a digital transmission system.
2. Background Description
In both analog and digital transmission systems, service channels carry voice (order wire), supervisory, alarm and control information between repeater locations and end terminals. The control and supervisory facilities require at least one master station (usually located at an end terminal) and slave stations (usually located at each repeater location). Status reporting from each repeater location often is accomplished by polling techniques wherein the supervisory equipment for each repeater station is assigned a unique address. When a repeater station receives a status request along with its particular address, it responds by transmitting the desired status information. The polling can be accomplished sequentially, in which case the information is regularly transmitted to the master station.
Usually service channel signals are connected to a digital transmission system at the VF (voice frequency) interface via an analog hybrid system (sometimes referred to as an analog branching unit) as illustrated in FIG. 1. Unfortunately this technique causes the through path service channel signals to be subjected to multiple A/D and D/A conversion (i.e. analog-todigital and digital-to-analog conversion) at each repeater location. Such conversion occurs independent of whether anything is dropped out or inserted at the repeater location. Such an operation degrades the overall service channel performance and adds an expense to the system.
It is a principal object of this invention to avoid a dual D/A and A/D conversion of digital service channel through path signals by performing the 3-way branching at a repeater in the digital domain.